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WASHINGTON (CNN) -- Supreme Court Justice Sandra Day O'Connor, announced her resignation Friday and launched the high-stakes battle over who will replace her.

The 75-year-old moderate conservative, who was the first woman to serve on the court, has been the swing vote on some of the highest-profile cases -- including abortion and vote-counting in the 2000 presidential election.

In a letter to President Bush, O'Connor said she needed to spend more time with her husband and would leave her post after the confirmation of her successor.

Her husband of more than 50 years, John J. O'Connor, has been suffering from the early stages of Alzheimer's.

This is the first Supreme Court vacancy since 1994, when President Clinton nominated Stephen Breyer. President Reagan appointed O'Connor who took her seat as associate justice on September 25, 1981. (Profile)

O'Connor's move did not surprise court watchers and portends a tremendous political battle. Already, conservative and liberal groups have been pushing the White House to fill a potential vacancy with someone who leans toward their points of view. (Tough fight expected)

Within an hour's notice of O'Connor's departure, Bush vowed in brief statement delivered in the Rose Garden to announce a nominee "in a timely manner."

And he sent a message to Democratic lawmakers not to filibuster his choice as he called for a "dignified" confirmation process that includes a "fair vote."

The White House said Bush will announce a nominee after he returns from next week's G8 summit.

A day's notice

On Thursday, Bush received word from the head marshall at the high court that one of the justices would have a letter to deliver the next day.

On Friday, the president learned O'Connor was the justice who would be retiring, the White House said. After that notice, Bush and O'Connor had a five-minute phone conversation.

There has been much speculation that Chief Justice William Rehnquist may retire, but O'Connor's move could have an even greater impact.

While Rehnquist falls regularly on conservative sides of issues, O'Connor has been unpredictable. If her replacement is a staunch conservative, the new justice could alter the court's ideological balance and affect decisions on the most contentious issues -- including abortion.

O'Connor has dismissed the swing vote label, telling CNN recently, "That's something the media has devised as a means of writing about the court, and I don't think that has a lot of validity."

But her votes have set the jaws of those on the left and the right.

She angered social conservatives by re-affirming abortion rights in several abortion-related cases, insisting states place "no undue burden" on those rights. Earlier this week, she sided firmly against Ten Commandments displays in both Texas and Kentucky.

O'Connor has also angered liberals, supporting limits on affirmative action and, in 2000, serving as the swing vote that ended vote counts in the presidential election, giving Bush a victory.

Finding a replacement

In preparation for a high court resignation, the White House previously held secret meetings about possible replacements, and senior officials have interviewed some possible candidates, Bush aides and advisers have said.

Bush has long been prepared to move very quickly on court vacancies, the White House said. (Potential nominees)

Friday, Bush said, "The nation deserves and I will select a Supreme Court justice that Americans can be proud of.

"The nation also deserves a dignified process of confirmation in the United States Senate, characterized by fair treatment, a fair hearing and a fair vote," he said. "I will choose the nominee in a timely manner so that the hearing and the vote can be completed before the new Supreme Court term begins."

The term begins in October.

Bush's comments were a reference to the recent confirmation battle over his judicial nominees. Democrats threatened filibusters, which a deal struck among seven Republicans and seven Democrats eliminated. But that deal left open the possibility of future filibusters.

For months, lawmakers on both sides of the aisle have said they expect a high-stakes battle when it comes time to fill a vacancy. Liberal and conservative groups have already launched major campaigns calling for nominees who lean toward their points of view.

While Bush has expressed opposition to abortion rights -- and the Republican Party, in its platform, opposes them -- a large majority of Americans supports them. A poll taken earlier this week found 65 percent of Americans want a new Supreme Court justice to uphold the Roe v. Wade decision, which established abortion rights, while only 29 percent want the decision overturned.

O'Connor, a Stanford graduate, wrote in her letter that she will leave "with enormous respect for the integrity of the court and its role under our constitutional structure."